A method can be seen in the DE 196 22 685 A1, in which signals are exchanged between an evaluating unit serving as a central unit and an ignition device as a module. For the transmission of these signals from the evaluating unit to the ignition device, an alternating voltage signal is generated, which is superposed on a direct voltage that serves for the voltage supply. For replies from the ignition device, its current input is varied in a targeted manner (see col. 6, beginning at line 25 and claims 6 to 9 of the DE 196 22 685 A1). A disadvantage of this arrangement is that only exactly one module can exchange signals with the central unit. Moreover, a corresponding method for the bi-directional data transmission can already also be seen from the DE 39 03 377 A1. Also therein, only exactly one module embodied as a counter can be coupled to the central unit embodied as a read-write device. The quiescent or resting current of the module must lie within a tightly limited prescribed magnitude range in order to be able to receive the current-modulated return signals.
Moreover, in the DE 44 11 184, there is described a motor vehicle safety system with a time-alternating energy and data transmission on a bus line.
A method for the signal transmission between a plurality of modules by superposition of an alternating signal on a direct voltage or a corresponding modulation thereof, can be seen in the U.S. Pat. No. 4,463,341 for example, in which a plurality of transmitters and receivers is arranged in a bus system, which use the direct voltage on the bus system for the voltage supply and superpose frequency modulated alternating voltage signals thereon, whereby respectively one transmitter and one receiver are calibrated to a common transmission frequency. Such a bus system could be readily transferred to the signal transmission with a central unit and several modules, in that the central unit would use alternating voltage signals with the respective transmission frequency. This, however, just like the U.S. Patent, would result in the disadvantage that an extremely high accuracy of the frequencies is required for the demodulation. The expense and effort required for such a bus system is considerable.
A bus system can be seen in the U.S. Pat. No. 4,736,367, wherein similarly a plurality of modules are supplied with voltage from a microcomputer over a bus line, and data are transmitted to the modules by superposed voltage signals, and furthermore, the modules transmit data back to the microcomputer by variation of their current input. For that purpose, a controllable current sink with a constant current source is provided respectively in the modules, so that each module can achieve a current input determined by the constant current source. Thereby, the differentiation of the data of the individual modules is achieved by a serial transmission in prescribed time windows, to which the modules are allocated by addresses. Due to the varying current input, a voltage signal arises at the microcomputer, which voltage signal is evaluated by means of a threshold value. In this context, it is found to be problematic that on the one hand the direct supply voltage and on the other hand the current input through the modules is known and constant for the recognition of the current modulated signals of the modules. For example, if modules with a different current input were to be used, then a comparison with the threshold value would be impossible. The addition of several new modules would also lead to errors, because, due to the current input of the added modules, it would similarly lead to a shifting of the total current input. If one further considers the fluctuations of the supply voltage in the usually battery-supplied bus systems in motor vehicles, as well as the temperature influences on the electrical components, it becomes clear that a great number of interfering values interfere with the signal transmission and can make a return recognition of the signal impossible.